In a mobile electronic device such as a mobile or cellular telephone for example, there is a need for improvement of the efficiency of an electric power supply for a longer battery run time.
In Japanese Patent Application Publication JP H 11-41825-A published on Feb. 12, 19999, Matsumura describes a power supply switching device for switching power supplies in accordance with a load consumption power. The power supply switching device has a battery provided in a mobile device, and has at least one constant voltage means. The power supply switching device includes a DC voltage conversion means for developing a DC voltage lower than the battery voltage, switching means for switching between the battery voltage and the DC voltage developed by the DC voltage conversion means to provide a power to the input of the constant voltage means, and control means for controlling the switching means to select the DC voltage developed by the DC voltage conversion means in a main drive state with large power consumption and to select the battery voltage in a standby state with small power consumption.
In Japanese Patent Application Publication JP H 7-95765-A published on Apr. 7, 1995, Ishikawa describes a stabilized DC power supply device. In the stabilized DC power supply device, a series regulator is connected to an output of a chopper regulator for integration. Thereby, the stabilized DC power supply has a small size, reduces noise and a ripple, and has high efficiency and high output voltage accuracy. It requires a small number of external parts and small space, and can be easily designed.
In Japanese Patent Application Publication JP 2003-9515-A, Eguchi describes a power supply system. In a device using a battery as a power supply, such as a PDA, a mobile telephone and the like, a low dropout regulator and a DC/DC converter are used by switching them in accordance with a load current of the device to thereby convert the voltage at the high efficiency. For switching between the low dropout regulator and the DC/DC converter, the DC/DC converter is caused to operate asynchronously, and the low dropout regulator and the DC/DC converter are caused to operate simultaneously, whereby the voltage ripple in the switching operation is reduced and a power supply with a stable output voltage is provided. This provides the stable power supply output having the low ripple and high conversion efficiency even in the operation of the device with a low load current, while keeping high conversion efficiency.
The DC voltage conversion efficiency of a DC-DC (DDC) converter and an LDO regulator are described in Texas Instruments Japan Ltd., “Power-Supply IC Cookbook for Low Voltage Era”, Apr. 15, 2006, pages 11-13, CQ Publishing CO., Ltd.